Formation of alpha phase in the massive and feathery gamma-TiAl alloys during aging in the single alpha field

Precipitation of alpha phase in massive and feathery microstructures was st udied during aging of a Ti-48 pct Al-2 pet W-0.5 pct Si alloy in the single alpha field. It was found that the alpha phase mainly precipitates along t he gamma-plate interfaces as laths in the feathery structure, while it nucl eates at various sites in the massive structure in the form of idiomorphs a nd especially of plates. The gamma(m) --> alpha reaction proceeds by the gr owth of pre-existing ct precipitates and chiefly by the development of new alpha plates. The alpha plates are likely to originate from the splitting o f unit dislocations into Shockley partials and to grow by the diffusional l edge mechanism, which shows both diffusional and shear character. During ag ing, the stacking faults (SFs) in the massive gamma domains evolve into SF- shaped alpha precipitates through a transition gamma' phase.